Sheaths and catheters are commonly used, particularly in endoluminal procedures. It is important that such sheaths have a good longitudinal strength in order to ensure that the sheath can be pushed through a patient's vasculature and that the sheath and its contents do not compress longitudinally as the sheath is guided into position.
In addition to needing to have longitudinal strength, such sheaths and catheters must be sufficiently flexible so as to be able to follow the curves of the patient's vasculature, but also to be kink resistant. Kinking of the sheath can lead to damage of the contents of the sheath, inability to extract the contents within the patient and generally to an abortive medical procedure.
For this reason, sheaths in particular often have complex structures with embedded strengthening elements such as coils or metal braids. The risk of kinking is reduced by having the strengthening structure extend substantially for the whole length of the sheath.
Although strengthening elements can be effective in minimising the risk of kinking, they can lead to loss of flexibility and as a result loss of trackability, that is ability of the sheath to follow the curves in a patient's vessels. Furthermore, they can lead to increased sheath diameter particularly at the distal end of the sheath, which typically must house components which have a greater diameter than the other elements in the introducer assembly such as pusher catheters, guide wire catheters and the like.
Examples of known sheath structures can be found, for instance, in WO-2006/113863, US-2008/0132906, US-2009/0177260, U.S. Pat. No. 7,470,282, U.S. Pat. No. 5,725,571 and WO-2004/080504.